FIG. 1 illustrates an aircraft engine 3 of the unducted fan type, in which the invention can be used. Region 6 is shown in cross-sectional schematic form in FIG. 2, wherein contra-rotating turbines 9 (hatched) and 12 (plain) are driven by a hot gas stream 15 provided by a core engine (not shown in detail). The turbines 9 and 12, in turn, drive contra-rotating fan blades 18 and 21 shown in FIGS. 1 and 2. (The term "contra-rotating" means that turbines 9 and 12, as well as blades 18 and 21 to which they are attached, rotate in opposite directions, as shown by arrows 24 and 27 in FIG. 1.) A view of sub-region 6A in FIG. 2 is shown in FIG. 3, and in more detailed form in FIG. 4. The fan blades 18 in FIG. 3 are supported by a structure which is shown as a ring 22 in FIG. 4.
Turbine blades 28 in FIGS. 3 and 4 comprise the turbine stage indicated as 6B in FIG. 2. A casing 24 (shown also in schematic form in FIG. 2) surrounds the turbine blades 28. A carrier, schematically shown as a ring 22 in FIG. 4, supports the fan blades 18 in FIG. 3. During operation, the centrifugal loading of the fan blades 18 is reacted by the ring 22 as a hoop stress.
The actual structure used is not the idealized ring 22 shown in FIG. 4, but is what is termed a "polygonal ring," shown in schematic perspective form in FIG. 4A. The polygonal ring includes two types of sections: one type is the blade support section 22B, also shown in FIG. 5, and which includes a thrust bearing 67 which carries the centrifugal load imposed by the fan blade 18.
The blade support section 22B also includes an alignment bearing 22E which serves to maintain fan blade 18 in correct radial position. Both the thrust bearing and the alignment bearing allow the blade to change in pitch, as discussed below.
The other type of section is a connecting beam 22A which connects neighboring blade support section 22B.
One type of polygonal ring is described in U.S. Pat. No. 4,863,352--Hauser et al., entitled "Blade Carrying Means." This patent is hereby incorporated by reference.
As stated above, the ring 22 in FIG. 4 surrounds a turbine stage 6B. A space, indicated by distance 23, exists between the ring 22 and the casing 24 of the turbine stage. One reason for the space is that the geometries of the components flanking the space are determined by different factors. That is, the design of the fan system, including both blades 18 and cowl 27 in FIG. 2, is determined by factors which are different than those which govern the design of the turbine components, namely, those inward of arrow 31.
A second reason for the existence of the space 23 is that the ring 22 and the casing 24 in FIG. 4 expand differently under centrifugal load. A third reason is that the ring 22 and the casing 24 are subjected to different temperatures during operation. The casing experiences a temperature of about 1,200.degree. F., while the ring 22 experiences a significantly lower temperature of about 500.degree. F. Consequently, the casing 24 expands more than does the ring. In one instance, distance 23A in FIG. 6A was 0.140 inches when all components were at room temperature. At aircraft takeoff, distance 23A shrank to 0.070 inches.
In the event that one of the fan blades 18 should break free during operation, the event shown in FIG. 4B occurs. The centrifugal load imposed on the ring 22 prior to breakage, indicated by arrow 30, suddenly vanishes, indicated by breaking 33. The inward load indicated by arrow 36 which formly balanced the centrifugal load 30 is now unbalanced, and the ring 22 almost instantly deforms into phantom shape 39 and strikes the casing 24. The force of impact between the ring 22 and the casing 24 can be ascertained from a simple example.
It is assumed that the fan diameter, dimension 42 in FIG. 1, is 12 feet. By way of example, it is assumed that each fan blade can be treated as a point mass of weight 54 pounds shown in FIG. 1 by weight 45, and located on the circumference of a circle 46 which is six feet in diameter. It is also assumed that the speed of rotation is 20 revolutions per second, or (2) (pi) (20) radians per second, i.e., about 126 radians per second.
Centrifugal acceleration is w.sup.2 r, wherein w is angular velocity (radians per second) and r is radius. In this example, the acceleration is about 47,628 feet per second per second: EQU 47,628=126(1/sec).times.126(1 sec).times.3(feet)
Dividing this number by the acceleration due to gravity, 32.2 feet per second per second, gives a quotient of about 1471. The quotient is the g field experienced by the point masses. Stated another way, each point mass 45 (representing the weight of each blade), which originally weighed 54 pounds, now weighs 80,000 pounds under centrifugal force (1471.times.54=80,000). Therefore, the balancing force 36 in FIG. 4B is also 80,000 pounds.
When the blade breaks free, the balancing force now acts unopposed in pulling the ring 22 toward phantom position 39. To a first order approximation, the situation is similar to dropping a weight of 80,000 pounds onto the casing 24.
The impact of the ring at region 47 in FIG. 4B can cause damage to the casing 24. More importantly, the deformation of the ring 22 can cause distortion to, and seizure in, the pitch change mechanism, which was not previously discussed.
The pitch change mechanism is that which alters the pitch of the fan blades 18; that is, the mechanism rotates the fan blades about axis 50 in FIG. 3 in order to change the pitch angle of the fan blades. The detailed operation of the mechanism need not be understood here, only that the mechanism must not be permitted to strike the casing 24 because impact with the casing 24 can cause the mechanism to deform, jam or render the mechanism inoperable. The pitch change mechanism must remain operational in order to pitch the remaining blades safely and to prevent overspeed of the propeller with further damage.
Further, the deformation indicated by phantom ring 39 in FIG. 4B is not desirable because the deformation moves the remaining fan blades from their original positions, thus displacing the centroid from its original position. The displacement causes significant imbalance and vibration.
Still further, deformation can break the ring.